The present invention relates to industrial computerized tomography (ICT) and particularly to the construction of two-dimensional, cross sectional, computerized tomography (CT) images of objects with the aid of a priori information regarding the objects.
ICT inspection of industrial products is a powerful quality assurance tool for determining if the internal characteristics of the manufactured products are in accordance with design specifications and for detecting any internal flaws. For example, in the manufacture of gas turbine engines, there are certain critical parts, such as turbine blades that are intricately cast with internal passages, whose internal characteristics are of paramount importance if safe, long term engine Performance is to be achieved. The passage walls must be of design thickness and the existence of internal flaws, such as cracks, voids, microshirks, porosities, coldshuts, passage blockages, etc., must be within design tolerances if the part is to withstand the rigors of an engine operating environment.
To perform an ICT inspection, the part is positioned between an X-ray source and a detector typically consisting of an array of detector elements aligned with a highly collimated, fan-shaped beam of X-rays projected by the source at a transverse slice of the part. The part is rotated in steps through an angle of at least 180.degree. plus the fan beam included angle, if possible, and the part slice is irradiated from a multiplicity of different projection angles. The attenuation of the X-ray flux is detected by each detector at each projection angle, and a computer reconstructs an image of the slice cross section from the X-ray attenuation signals generated by the detector elements at each projection angle. For certain part geometries, the X-ray path length through certain slices thereof can be so great at some projection angles that X-ray penetration is insufficient to generate meaningful attenuation data. In other cases, certain projection angles may be obstructed by the presence of associated objects. Obviously, the inability of obtaining meaningful X-ray attenuation data throughout the complete scanning range degrades the quality of the reconstructed slice cross section image. The image is marred by artifacts, and the internal characteristics of the part become ill-defined and blurred to the point that internal flaws become difficult to detect.
To improve CT image quality, resort to a priori or known information about the object under inspection has been proposed. In the commonly assigned application of K. C. Tam entitled "Method to Obtain Object Boundary Information in Limited-Angle Computerized Tomography", Ser. No. 032,804, filed Apr. 1, 1987, now U.S. Pat. No. 4,888,693 the disclosure of which is specifically incorporated herein by reference, the exterior boundary of the test object is estimated. This is accomplished by performing a low energy X-ray exposure of the test object at each CT projection or scanning angle to determine the object edges in each instance. The low energy X-ray data generated by the detector elements are computer processed as a succession of curve fittings at the object edges to construct a polygon-shaped region approximating the convex hull of the object. If the object is basically convex in shape, such as cylindrical, the convex hull corresponds quite closely to the object boundary. This convex hull information is then used to correct the CT image reconstruction process by setting to zero those pixels known to be beyond the object boundary, thereby achieving a high quality image of the slice cross section.
Unfortunately, not all test objects are convex in shape. Many have significantly concave boundary features. Consequently, the convex hull then becomes merely an approximation of the object boundary, which diminishes in value as the degree and number of concave boundary features increases.
It is accordingly an object of the present invention to provide a method for improving the quality of CT image reconstruction.
A further object is to provide a method of the above-character, wherein more precise a priori information regarding the test object is utilized in CT image reconstruction.
An addition object is to provide a method of the above-character, wherein physical measurements of the test object boundary are utilized as a priori information in the reconstruction of CT images.
Other objects of the invention will in part be obvious and in part appear hereinafter.